Equilibrated charging method for a lithium-ion or lithium-polymer battery

ABSTRACT

An equilibrated charging method for n cells of a lithium-ion or lithium-polymer battery, connected in series. The method is characterised in carrying out a continuous monitoring of the levels of charge of the different cells ( 1 ), from the beginning of the operation of charging the battery ( 2 ) and during the process and, as a function of the analysis of the levels of charge, to carry out a uniform supply to all the cells ( 1 ), or an equilibration of the levels of charge of the cells ( 1 ), by supplying the same in different manners, as a function of the levels of charge thereof.

The present invention relates to the field of charging or the charge ofrechargeable batteries, and has for its object a process for charging orthe balanced charge of cells of a lithium ion or lithium polymerbattery.

Optimum electrical charging of batteries comprising several constituentcells, poses problems difficult to solve, particularly when the numberof elements or cells in series is high.

In the case of a lithium ion or lithium polymer battery, there is addedto these problems the optimization of the charge of the differentelements or cells, risks of irremediable deterioration of said elementsor said cells in the case of overcharging, particularly by overheatingor over-voltage.

It is known on the one hand that in batteries which use lithium ion orlithium polymer elements in series, the performances of capacitance ofeach element or cell after charging are not identical and that thesedifferences increase from cycle to cycle of charging and discharginguntil the end of the life of the battery in question.

It is known, on the other hand, that the lithium ion and lithium polymerbatteries cannot be overcharged on the occasion of charging, norundercharged on the occasion of use (discharge). The value of maximumretained voltage, by way of example and not in a limiting sense, for theovercharge for each of the elements of a lithium ion and lithium polymerbattery in series, is 4.20 volts and the retained tension to stopdischarge, and thereby avoid degradation of the battery, is 2.70 volts.

It is generally that, for each of the lithium ion or lithium polymerelements or cells, the voltage at the terminals of the element or of thecell is the image of the capacitance stored in the element or the cellin question. This indication of voltage does not give the precise valueof capacitance in ampere/hours or in watt/hours, but gives a percentageof the capacitance of the element in question at the time of measuringthis voltage.

FIG. 1 of the accompanying drawings shows a curve showing thedevelopment of the voltage at the terminals of a lithium ion elementrelative to its capacitance (in the case of the discharge curve withconstant current, the time is proportional to the percentage of thecapacitance stored in the lithium ion element in question, wherein: 0sec=>95% (4.129 volts), 6.150 seconds=>50% (3.760 volts) and 12.300seconds=>0% (3.600 volts). It should be noted that over an importantportion of this curve, the capacitance is substantially linear beforerapidly degrading. To control the operations of charge and discharge ofa lithium ion element or cell, operation is conducted in thesubstantially linear portion, which permits affirming that the voltageis the image of the capacitance.

Given the indications developed in the three preceding points, it can beverified that, in a battery constituted by more than three to fourlithium ion or lithium polymer elements in series, the charging of thebattery will be stopped when the most highly charged element will havereached 4.20 volts and, conversely, during discharge, the latter will bestopped when the element of least capacitance will have reached thevoltage of 2.70 volts: it is hence the element which has the lowestcapacitance which determines the overall capacitance of the battery.This permits understanding that, when the battery has a large number ofelements in series, the risk of not using all of the capacitance of thebattery is real, because it is the least capacitative element whichdetermines in a limiting manner the total capacitance of the battery.Moreover, this phenomenon worsens as the charging/discharging cyclesaccumulate.

This phenomenon of unbalanced charging is essentially caused by thedifferences of capacitance and of internal resistance between theconstituent elements of the battery, these differences resulting fromthe variation of the quality of production of the lithium ion or lithiumpolymer elements.

So as to optimize the capacitance of the battery over time, which isvery important for the cost of use, it is necessary to overcome theproblem stated above by providing, before stopping the charge, arebalancing of all the elements or all the cells of the battery. Thisbalancing will permit charging 100% of all the elements no matter whattheir capacitance.

In the present state of the art, this balancing takes place at the endof charging, by deriving the charge current of the element charged to100%, which is to say when this latter has reached a voltage of 4.20volts. Thus, the elements are thus stopped when they reach 4.20 voltsand there is thus obtained a charge of 100% of all the elements at theend of the charging operation.

But this known balancing technique at the end of charging has notabledrawbacks.

Thus, these balancing systems require resistances of important magnitudeto be able to dissipate the consequent currents, and this the more so asthe balancing system enters into action when the charge currents aregreater still, which takes place when the elements of the battery arevery unbalanced.

Moreover, this great dissipation of power gives rise to a consequentincrease of temperature, which can be troublesome in the case of compactbatteries integrating derivative resistances.

Moreover, it could happen that, despite the injection of large chargingcurrents toward the end of the charging operation, the battery will notbe balanced when the end of charging condition is fulfilled.

Furthermore, in high power applications, the recharging time of thebattery, particularly for complete recharging, is long, even very long.It thus frequently happens that the effective charging time between twophases of discharge will be too short to terminate the chargingoperation, and the charge is thus interrupted whilst the unbalancesbetween the elements or cells are still not compensated (in the case ofthe presence of a balancing system at the end of charge according to theprior art). The repetition of these phenomena gives rise to rapiddegradation of the performance of the battery in question.

The present invention has for its object to propose an optimizedcharging solution, having the mentioned advantages and overcoming thepreviously mentioned drawbacks with respect to the state of the art.

To this end, the invention has for its object a process for balancedcharging of n cells, with n≧2, constituting a lithium ion or lithiumpolymer battery and associating in series, each cell being comprised ofone or several elements mounted in parallel, this processing beingcharacterized in that it consists in continuously providing, from thebeginning of the charging operation of the batteries and throughout thecourse of this operation, a surveillance of the charge levels of thedifferent cells, and in carrying out, as a function of said evaluationof said charge levels, either a uniform supply of all the cells, or abalancing of said levels of charge of said cells by supplying theselatter in a differentiated manner as a function of their current chargelevels.

The steps of the process mentioned above can be carried out in twodifferent manners, according to two technologically differentimplementations.

Thus, by using a solution based essentially on an analog technology, thesurveillance of the charge levels is carried out in a continuous mannerand the differentiated supply is carried out as soon as, and as long as,the differences of charge level, between the most and least chargedcells, exceeds a predetermined threshold value.

As a modification, using a preferred solution which uses a digitalprocessing of the signals and a management of the process by a digitalprocessing unit, the surveillance of the charge levels is carried out byrepeated measurements and the differentiated supply applied during apredefined time, in case of verification of the conditions of thisequilibrium of the required charge levels.

This second solution permits simplifying both the material and thesoftware implementations necessary for practicing the process.

As to this second solution, the process consists preferably intriggering for each cell of the battery, one after the other, in asequential manner during fractional duration of the total charge time ofthe battery, sequences comprising a refreshed evaluation of the chargelevel of the cell in question, followed, as a function of its chargelevel and in relation to all the charge levels of the other cells of thebattery, a uniform or differentiated supply, this according to arepetitive cycle all during the charging operation.

According to an advantageous embodiment of the invention, said processcomprises at least the execution of the following operations under themanagement of a digital processing unit, and this from the beginning ofcharging:

-   -   evaluation, preferably at regular intervals, of the quantity of        energy stored in each cell by measuring an indicative parameter        of said quantity;    -   comparative analysis of the different quantities of energy        evaluated or the different values of the measured parameter;    -   determination of the tardiest cell to charge, and, as the case        may be, of the cell or cells most advanced in charging;    -   supplying the different cells mounted in series in a uniform        manner or with limitation of the charge current for the cells        other than the most retarded or most advanced, by derivation of        all or a portion of said current at the level of this or these        latter;    -   sequential repetition of the different said operations until        there is obtained an end of charge condition of the battery or        the detection of a default, of a dysfunction or of the exceeding        of a permissible threshold value.

The experiments and work of the applicant have shown that thissequential balancing process distributed all during the charge permitshaving all the elements or cells constituting the battery charged at thesame percentage at a given instant of charging, and thus to achieve 100%capacity for all the elements constituting the battery at the end ofcharging, and this independently of their own capacitance.

The invention will be better understood, from the following description,which relates to a preferred embodiment, given by way of non-limitingexample, and explained with reference to the accompanying schematicdrawings, in which:

FIG. 2 of the accompanying drawings is a synoptic diagram of a devicefor practicing the process of the invention;

FIG. 3 is a more detailed diagram of the device shown in FIG. 2,according to a modified embodiment of the invention;

FIG. 4 is an ordinogram showing schematically the different steps of theprocess according to one embodiment of the invention (in thisordinogram, it is necessary to understand by the term “element”, anelement or a cell with several elements in parallel) and,

FIG. 5 shows chronograms illustrating by way of non-limiting example,for a battery of twelve cells, the operations carried out during acharging cycle with balancing according to the process of the invention.

This latter has for its object a process for charging or balancedcharging of n cells 1, wherein n≧2, constituting a lithium ion orlithium polymer battery 2 and associated in series, each cell 1 beingcomprised by one or several elements mounted in parallel.

According to an advantageous embodiment of the invention, this processcomprises at least the execution of the following operations under themanagement of a digital processing unit, and this from the beginning ofthe charge:

-   -   evaluation, preferably at regular intervals, of the quantity of        energy stored in each cell 1 by measuring a parameter indicative        of said quantity;    -   comparative analysis of the different quantities of energy        evaluated or the different values of the measured parameter;    -   determination of the cell 1 most tardy in charging and, as the        case may be, of the cell or cells 1 the most advanced in        charging;    -   supplying the different cells 1 mounted in series in a uniform        manner or with limitation of the charge current for the cells 1        other than the most retarded or the most advanced in charging,        by derivation of all or a portion of said current in this or        these latter;    -   sequential repetition of the different said operations until        there is obtained an end of charging condition of the battery 2        or the detection of a fault, or a dysfunction or the exceeding        of an admissible threshold value.

Preferably, the parameter measured in each cell 1 and used for theevaluation of the quantity of energy stored in this latter, is thevoltage at the terminals of the cell 1 in question.

As indicated above, the limitations of the charging current can ifdesired affect all the cells in advance of charging relative to theleast charged cell, as the case may be with different degrees of supplylimitation.

However, so as further to spread out the active balancing phases, theinvention preferably provides that only the cell or cells of the mostadvanced charging condition (during a given fractional duration n), willbe subjected to limitation of charge (during the following fractionalduration n+1). Thus, the cells whose charge level is only slightlygreater than that of the least charged cell, will continue their normalcharging.

The discrimination between the cells subjected to temporary limitationof the charge and those which are not (during a fractional time of thetotal charging time), can for example derive from the situation (interms of values) of the levels of charge of these cells relative to agiven threshold value by [value of the least charged cell+delta (Δ)].

Moreover, by adopting the strategy of limiting the charge current of themost charged cells all during the charging of the battery, instead ofwaiting to the end of said charge, the invention permits avoiding anyrisk of overheating of the battery 2 because of late balancing andguarantees balanced voltages in the cells 1 at the end of charging.

Furthermore, by beginning the balancing as soon as the charge begins andby pursuing its action throughout the operation of charging, it ispossible to guarantee a substantially balanced battery all during thecharging operation, which is to say even in the case of interrupting ofcharging before its normal conclusion.

According to a preferred characteristic of the invention, the derivationof current in the most advanced cell in charging, is carried out bymeans of derivation circuits 4 each associated, by mounting in parallel,with one of said cells 1 (a circuit 4 for each cell 1), said circuits 4each integrating a switching member 5 and, as the case may be, at leastone component for dissipating electrical energy 6, if desiredadjustable, such as for example an electrical resistance (FIGS. 2 and3).

The switching member 5 could for example be selected from the groupcomprised by electromagnetic or electronic relays, bipolar transistorsor with field effect or the like.

Moreover, the derivation of energy connected with balancing the chargesof the different cells 1 being distributed over all the duration ofcharging, the switching component 5, as well as the dissipationcomponent 6 associated therewith, can be optimized.

According to a preferred embodiment of the invention, the charging withsequential balancing consists more precisely in carrying out, byrepeating over all the charging of the battery 2, the followingoperations:

-   -   a) scrutinizing one by one all the cells 1 of the battery 2 by        measuring the voltages at their terminals, this without the        resistances 6 of derivation or balancing being connected;    -   b) detecting the cell 1 which is tardiest to charge;    -   c) detecting the cells 1 which, relative to the cell 1 the        tardiest to charge or the least charged, have an overcharge        greater than a predetermined threshold value of spacing of        capacitance, for example corresponding to a difference of        voltage (dVs) of 10 mV;    -   d) individually connecting each cell 1 detected to have an        overcharge greater than the threshold value, to a corresponding        balancing resistance 6, so as to lead to decrease of the        charging current for each of the cells 1 in question, for        example by about 10%, during a predetermined sequential time,        for example two seconds;    -   e) disconnecting the balancing resistances 6 of all the cells 1        after lapse of the predetermined sequential duration;    -   f) repeating the steps a) to e) after elapse of a stabilization        delay of the voltages of the cells 1.

The charging of the battery is stopped normally when the intensity ofthe overall charging current of the assembly of cells of this latterfalls below a predetermined threshold value, for example 50 mA.

By way of example of a way to practice the invention, the powers of thedifferent derivation circuits 4 are selected to be near the valuesprovided by the following formula:

${{Psd}\mspace{14mu} \max} = \frac{V\mspace{14mu} \max \mspace{14mu} {cell}*\%*{AH}}{Tc}$

in which:

-   -   Psd max=maximum power optimized to dissipate, expressed in        watts;    -   Vmax cell=maximum voltage measured during charging at the        terminals of a cell, expressed in volts;    -   %=ratio expressed in percentage, corresponding to the maximum        spacing between two cells that it is desired to make up for        during charging;    -   AH=nominal battery capacitance expressed in Ah (Ampere-hour);    -   Tc=time of battery charge expressed in hours.

Moreover, to provide precise and progressive regulation of the chargingof each cell 1, the voltage at the terminals of each cell 1 is measuredprecisely by an assembly 7 of corresponding measuring modules 7′, whoseoutput signals are transmitted, preferably after digitization, to thedigital processing unit 3, this latter controlling, in the followingcycle, the switching members 5 of the different derivation circuits 4 asa function of the comparative development of said output signalsprovided by the modules 7′.

According to a very advantageous embodiment of the invention, referringby way of example to FIGS. 4 and 5, the operations are repeated, duringeach charging operation in a cyclic loop formed by two operational halfcycles, executed successively in each cycle loop, a first half cyclecomprising the consecutive execution of the following operations:successive reading of the voltages of the different cells 1 andtriggering, offset in time, the balancing resistance 6 for each cell 1whose difference of voltage (dV) with the cell 1 that is tardiest tocharge of the preceding cycle, is greater than a threshold value (dVs),and the second half cycle comprising the following operations:successive disconnection of the balancing resistances 6 of the differentcells 1 and awaiting the stabilization of the voltages of differentcells 1 before their reading during a first half cycle of the followingcycle, the two half cycles having preferably durations substantiallysimilar, for example about 2 seconds.

Thanks to the cyclic repetitions of the operations of the two halfcycles (with a cycle duration for example of 4 seconds), during all theprocedure of charging the battery 2, that is until the occurrence of anend of charging event or of safety information, all the cells 1 (and theelement or elements comprising each of these latter) have at any time alow dispersion of capacitance (because of the connections of constantcharge between the cells) and recovering in optimum manner their maximumperformances.

Moreover, the process according to the invention permits accepting atthe beginning of charging the great charge differences between cells 1,and the “adjustment” or balancing being distributed over the entireduration of the charging procedure of the battery 2.

According to a first modification, it can be provided that the thresholdvalue of difference of voltage dVs consists in a first predeterminedfixed value V1, for example 10 mV, if the voltage difference dV betweenthe voltage of the cell 1 having the highest voltage and the voltage ofthe cell 1 having the least voltage, is less than a second predeterminedfixed value V2, greater than the first predetermined threshold value V1,for example 100 mV.

Moreover, it can thus also be provided that, if the voltage differencedV between the voltage of the cell 1 having the highest voltage and thevoltage of the cell 1 having the lowest voltage is greater than a secondpredetermined fixed value V2, for example 100 mV, the threshold value ofvoltage difference dVs consists in a third predetermined fixed value V3less than said second value V2, for example 30 mV.

Preferably, the third predetermined fixed value V3 is greater than saidpredetermined fixed first value V1.

According to a second modification, it can, in an alternative manner, beprovided that the threshold value of the difference of voltage dVscorresponds to a given fraction of the difference of voltage dV,measured during the preceding cycle, between the voltage of the cell 1having the highest voltage and the voltage of the cell 1 having thelowest voltage, if during the cycle in question, said difference ofvoltage dV is again greater than a fourth fixed predetermined value V4,for example 10 mV.

Advantageously in each of the two mentioned variations, and as alreadymentioned above, the measurements of the voltages of the different cells1 are taken only after the elapse of a given delay, for example 2seconds, following the suppression of the current derivatives, so as topermit a stabilization of the voltages at the terminals of said cells 1.

So as to preserve the cells 1 of the battery 2 from possible exposure toover-voltage, the management program of charging, whose ordinogram canfor example correspond to that shown in FIG. 4, can comprise theexecution of a certain number of tests before the beginning of thecharge and in the course of and at the end of charging.

Thus, the charging process can consist, at the outset, before startingexecution of the operations, in measuring the no load voltage Vo of thecharger 8 connected to the battery 2 as to its charge, and in stoppingsaid charging process, with possible triggering of a corresponding alarmand/or display of a message, if said no load voltage Vo is greater than[n×maximum permissible voltage Vmax for each cell 1].

Similarly, said process can also consist, before execution of a loop ora following cycle, in verifying whether at least one of the cells 1 ofthe battery 2 has at its terminal a voltage greater than the maximumpermissible voltage Vmax (for example and not in a limiting sense 4.23V) and, in the affirmative, interrupting the charging process, ifdesired with triggering of a corresponding alarm and/or display of amessage.

The present invention also has for its object a device for practicingthe process described above, of which the principal constituent elementsare shown schematically in FIGS. 2 and 3.

This device is essentially constituted, on the one hand, by an assembly7 of modules 7′ for measuring the associated voltage at one of the cells1 in series forming the battery 2 and measuring the voltages at theterminals of these latter, on the other hand, by a plurality ofderivation circuits 4 each mounted in parallel with the terminals of acorresponding cell 1 and being adapted to be opened and closedselectively, and, finally, by a unit 3 for digital processing andmanagement of the process, said unit 3 receiving the measurement signalsfrom said assembly 7 of modules for measuring the voltage 7′ andcontrolling the condition [closed/opened] of each derivation circuit 4.

The module 7′ will consist for example in circuits for differentialmeasurement of voltage with an operational amplifier, with a precisionof measurement of at least 50 mV.

Preferably, each derivation circuit 4 comprises a switching member 5,forming a switch and whose condition is controlled by the digitalprocessing unit 3 and, as the case may be, at least one component 6 fordissipation of electrical energy, such as for example one or moreresistances.

As shown in FIG. 3 of the accompanying drawings, and according to apreferred embodiment of the invention, the assembly 7 of modules 7′ formeasuring the voltage comprises, on the one hand, n analog modules 7′for measuring voltage, each associated directly with a cell 1 of thebattery 2, and on the other hand, in a multiplexer circuit 9 whoseinputs are connected to the outputs of said module 7′, and, finally, ananalog/digital converter circuit 10 connected at its input to the outputof the multiplexer circuit 9 and at its outlet to the digital processingand management unit 3.

As to a preferred application, and without limitation of the invention,the device shown in FIGS. 2 and 3 could desirably be integrated into aself-contained electric power tool assembly.

In this connection, it should be noted that the derivation circuits 4associated individually with the cells 1 of the battery 2, could also beused if desired to adjust the charges of said cells 1 to a levelcompatible with long time storage, without use, of said battery 2.

Of course, the invention is not limited to the embodiments described andshown in the accompanying drawings. Modifications remain possible,particularly as to the construction of the various elements or bysubstitution of technical equivalents, without thereby departing fromthe scope of protection of the invention.

1-21. (canceled)
 22. Process for the balanced charging of n cells, withn≧2, constituting a lithium ion or lithium polymer battery andassociated in series, each cell being comprised by one or severalelements mounted in parallel, which process is characterized in that itconsists in providing continuously, preferably from the onset of thecharging operation of the battery (2) and throughout the course of thisoperation, a surveillance of the levels of charge of the different cells(1), and in carrying out, as a function of the mentioned evaluation ofsaid charge levels, either a uniform supply of all the cells (1), or abalancing of said charge levels of said cells (1) by supplying theselatter in a differentiated manner as a function of their current levelsof charge, and in that it consists in triggering for each cell (1) ofthe battery, one after the other, in a sequential manner, for afractional portion of the total charge time of the battery (2),sequences comprising a refreshed evaluation of the level of the chargeof the cell (1) in question, followed, as a function of its level ofcharge and with respect to all the levels of charge of the other cells(1) of the battery, a uniform or differentiated supply, this accordingto a repeating cycle throughout the operation of charging.
 23. Processaccording to claim 22, characterized in that it comprises at least theexecution of the following operations under the management of a digitalprocessing unit (3), and this from the beginning of charging:evaluation, preferably at regular intervals, of the quantity of energystored in each cell (1) by measuring a parameter indicative of saidquantity; comparative analysis of the different evaluated quantities ofenergy or of the different values of the measured parameter for eachcell (1); determination of the cell (1) tardiest to charge and, as thecase may be, of the cell or cells (1) the most advanced in charging;supplying the different cells (1) mounted in series in a uniform manneror with the limitation of charging current for the cells (1) other thanthe tardiest or for the cell or cells most advanced in charging, byderivation of all or a portion of said current at the level of this orthese latter; sequential repetition of the different mentionedoperations obtaining an end condition of charge of the battery (2) orthe detection of a fault, of a dysfunction or an exceeding of anadmissible threshold value.
 24. Process according to claim 23,characterized in that the measured parameter in each cell (1) andutilized for evaluation of the quantity of energy stored in this latter,is the voltage at the terminals of the cell (1) in question.
 25. Processaccording to claim 23, characterized in that the derivation of currentin the cell or cells that are most advanced in charging, is carried outby a means of derivation circuits (4) each associated by mounting inparallel with one of said cells (1), said circuits (4) each integratinga switching member (5) and, as the case may be, at least one componentfor dissipation of energy (6), if desired adjustable, such as forexample an electrical resistance.
 26. Process according to claim 24,characterized in that charging with sequential balancing consists moreprecisely in carrying out, while repeating them during the course ofcharging the battery (2), the following operations: a) scrutinizing oneby one all the cells (1) of the battery (2) by measuring the voltages attheir terminals, this without the resistances (6) of derivation orbalancing being connected; b) detecting the cell (1) tardiest to charge;c) detecting the cells (1) which, relative to the least charged ortardiest cell (1), have an overcharge greater than a predeterminedthreshold value of difference of capacity, for example corresponding toa difference of voltage (dVs) of 10 mV; d) individually connecting eachcell (1) detected to have a surcharge greater than a threshold value, toa corresponding balancing resistance (6) so as to produce a decrease ofthe charging current for each of the cells (1) in question, for exampleof about 10%, during a predetermined sequential duration, for exampletwo seconds; e) disconnecting the balancing resistances (6) of all thecells (1) after lapse of the predetermined sequential duration; f)carrying out again steps a) to e) after the elapse of a stabilizationdelay of the voltages of the cells (1).
 27. Process according to claim22, characterized in that the charging of the battery (2) is normallystopped when the current intensity of the overall charge of the assemblyof cells (1) of this latter descends below a predetermined thresholdvalue, for example 50 mA.
 28. Process according to claim 25,characterized in that the voltage at the terminals of each cell (1) ismeasured precisely by an assembly (7) of corresponding measurementmodules (7′), whose output signals are transmitted, preferably afterdigitization, to the digital processing unit (3), this lattercontrolling, in the following cycle, the switching members (5) of thedifferent derivation circuits (4) as a function of the comparativedevelopment of said output signals provided by the modules (7′). 29.Process according to claim 23, characterized in that the operations arerepeated, during all the charging operation, as a cyclic loop formed bytwo operational half cycles, carried out successively at each cycleloop, a first half cycle comprising the consecutive execution of thefollowing operations: successive reading of the voltages of thedifferent cells (1) and triggering, offset in time, the balancingresistance (6) for each cell (1) whose voltage difference (dV) with thetardiest cell of the preceding cycle is greater than a threshold value(dVs), and the second half cycle comprising the following operations:successive disconnection of the balancing resistances (6) of thedifferent cells (1) and waiting for the stabilization of the voltages ofthe different cells (1) before their reading during the first half cycleof the following cycle, the two half cycles preferably havingsubstantially similar durations, for example about 2 seconds. 30.Process according to claim 29, characterized in that the threshold valueof voltage difference (dVs) consists in a first predetermined fixedvalue (V1), for example 10 mV, if the voltage difference (dV) betweenthe voltage of the cell (1) having the highest voltage and the voltageof the cell (1) having the least voltage is less than a secondpredetermined fixed value (V2), greater than the first predeterminedthreshold value (V1), for example 100 mV.
 31. Process according to claim30, characterized in that, if the voltage difference (dV) between thevoltage of the cell (1) having the highest voltage and the voltage ofthe cell (1) having the lowest voltage is greater than a secondpredetermined fixed value (V2), for example 100 mV, the threshold valueof voltage difference (dVs) consists of a third predetermined fixedvalue (V3) less than said second value (V2), for example 30 mV. 32.Process according to claim 31, characterized in that the thirdpredetermined fixed value (V3) is greater than said first predeterminedfixed value (V1).
 33. Process according to claim 29, characterized inthat the threshold value of the difference of voltage (dVs) correspondsto a given fraction of the voltage difference (dV), measured during thepreceding cycle between the voltage of the cell (1) having the highestvoltage and the voltage of the cell (1) having the lowest voltage, ifduring the cycle taking place, said voltage difference (dV) is stillhigher than a fourth predetermined fixed value (V4), for example 10 mV.34. Process according to claim 29, characterized in that themeasurements of the voltages of the different cells (1) are carried outonly after the elapse of a give delay, for example 2 seconds, followingthe suppression of the current derivations, so as to permitstabilization of the voltages at the terminals of said cells (1). 35.Process according to claim 26, characterized in that the powers of thedifferent derivation circuits (4) are selected to be near the valuesprovided by the formula:${{Psd}\mspace{14mu} \max} = \frac{V\mspace{14mu} \max \mspace{14mu} {cell}*\%*{AH}}{Tc}$in which: Psd max maximum power optimized to dissipate, expressed inwatts; Vmax cell=maximum voltage measured during charging at theterminals of a cell, expressed in volts; %=ratio expressed inpercentage, corresponding to the maximum difference between two cells tocompensate during charging; AH=nominal capacitance of the batteryexpressed in Ah (Ampere-hours); Tc=battery charge time expressed inhours.
 36. Process according to claim 23, characterized in that itconsists, at the outset, before triggering the execution of theoperations, in measuring the standby voltage (Vo) of a charger (8)connected to the battery (2) as to its charge, and stopping saidcharging process with if desired the triggering of a corresponding armand/or display of a message, if said standby voltage (Vo) is greaterthan [n×maximum admissible voltage (Vmax) for each cell (1)]. 37.Process according to claim 29, characterized in that it consists, beforethe execution of a following loop, in verifying whether at least one ofthe cells (1) of the battery (2) has at its terminals a voltage higherthan the maximum admissible voltage (Vmax) and, in the affirmative, ininterrupting the process of charging, if desired with the triggering ofa corresponding alarm and/or display of a message.
 38. Device for thepractice of a process according to claim 22, characterized in that it isessentially constituted, on the one hand, by an assembly (7) of modules(7′) for measuring the voltage associated each with one of the cells (1)in series forming the battery (2) and measuring the voltages at theterminals of these latter, on the other hand, by a plurality ofderivation circuits (4) each mounted in parallel with the terminals of acorresponding cell (1) and each being adapted to be opened and closedselectively, and, finally, by a digital processing unit (3) for managingthe process, said unit (3) receiving measurement signals from saidassembly (7) of measuring modules of the voltage (7′) and controllingthe condition [closed/open] of each derivation circuit (4), eachderivation circuit (4) comprising a switching member (5), forming aswitch whose condition is controlled by the digital processing unit (3)and, as the case may be, at least one component (6) for dissipation ofelectrical energy, such as for example one or more resistances. 39.Device according to claim 38, characterized in that the assembly (7) ofmodules (7′) for measurement of the voltage comprises, on the one hand,n analog modules (7′) for measuring the voltage, each associateddirectly with a cell (1) of the battery (2), on the other hand, in amultiplexing circuit (9) whose inputs are connected to the outputs ofsaid modules (7′) and, finally, an analog/digital converter circuit (10)connected at its input to the output of the multiplexer circuit (9) andat its output to the digital processing and management unit (3). 40.Device according to claim 38, characterized in that it is integratedinto an assembly of a self-contained electric power tool.